Endogenous and stimulated maturation of erythroblasts in vitro are similar in the following respects: both require erythropoietin since both are inhibited by antibody to erythropoietin; they require calcium in the external medium; are inhibited in the same way by inhibitors of DNA, ribosomal function, oxidations, and oxidative phosphorylation. Ionophores are stimulators of erythropoiesis and at certain concentrations are mitotic inhibitors in acidophilic erythroblasts. A protein very similar to or identical with that isolated from rabbit leukocytes which stimulates erythropoiesis in vitro has been isolated from beef spleen and purified. Lignoceric acid ceramide has been proved by gas chromatographic analysis and synthesis to be the active principle in the stimulator ethanol extract of leukocytes.